What are the differences between nested classes and inner classes in Java?

• Nested classes: Static members of an outer class. They do not have direct access to non-static members (fields and methods) of the outer class, only through an instance of the outer class. Declared with the static keyword.

• Inner classes: Non-static members of an outer class. They have full access to all members (static and non-static) of the outer class, including private ones. To create an instance of an inner class, an instance of the outer class is required. They do not use the static keyword.

Example:

// Outer class
class Outer {
    private int outerField = 10;
    private static int staticOuterField = 20;

    // Static nested class
    static class StaticNestedClass {
        void display() {
            // Access only static members of the outer class
            System.out.println("Static nested: " + staticOuterField);
            // No direct access to outerField
            // System.out.println(outerField); // Compilation error
        }
    }

    // Non-static inner class
    class InnerClass {
        void display() {
            // Access to all members of the outer class
            System.out.println("Inner: " + outerField);
            System.out.println("Inner (static): " + staticOuterField);
        }
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        // Creating an instance of the static nested class
        Outer.StaticNestedClass staticNested = new Outer.StaticNestedClass();
        staticNested.display();

        // Creating an instance of the inner class requires an instance of the outer class
        Outer outer = new Outer();
        Outer.InnerClass inner = outer.new InnerClass();
        inner.display();
    }
}

Feature	Nested Class	Inner Class
Keyword	static	No
Access to outer class members	Only static	All
Requires an instance of the outer class to create an instance	No	Yes
Connection with an instance of the outer class	No	Yes